Every
Apollo astronaut did it. They couldn't touch their noses to
the lunar surface. But, after every moonwalk (or "EVA"),
they would tramp the stuff back inside the lander. Moondust
was incredibly clingy, sticking to boots, gloves and other
exposed surfaces. No matter how hard they tried to brush their
suits before re-entering the cabin, some dust (and sometimes
a lot of dust) made its way inside.

Once
their helmets and gloves were off, the astronauts could feel,
smell and even taste the moon.

Right:
At the end of a long day on the moon, Apollo 17 astronaut
Gene cernan rests inside the lunar module Challenger. Note
the smudges of dust on his longjohns and forehead. Photo credit:
Jack Schmitt.

The
experience gave Apollo 17 astronaut Jack Schmitt history's
first recorded case of extraterrestrial hay fever. "It's
come on pretty fast," he radioed Houston with a congested
voice. Years later he recalls, "When I took my helmet
off after the first EVA, I had a significant reaction to the
dust. My turbinates (cartilage plates in the walls of the
nasal chambers) became swollen."

Hours
later, the sensation faded. "It was there again after
the second and third EVAs, but at much lower levels. I think
I was developing some immunity to it."

Other
astronauts didn't get the hay fever. Or, at least, "they
didn't admit it," laughs Schmitt. "Pilots think
if they confess their symptoms, they'll be grounded."
Unlike the other astronauts, Schmitt didn't have a test pilot
background. He was a geologist and readily admitted to sniffles.

Schmitt
says he has sensitive turbinates: "The petrochemicals
in Houston used to drive me crazy, and I have to watch out
for cigarette smoke." That's why, he believes, other
astronauts reacted much less than he did.

But
they did react: "It is really a strong smell," radioed
Apollo 16 pilot Charlie Duke. "It has that taste -- to
me, [of] gunpowder -- and the smell of gunpowder, too."
On the next mission, Apollo 17, Gene Cernan remarked, "smells
like someone just fired a carbine in here."

Schmitt
says, "All of the Apollo astronauts were used to handling
guns." So when they said 'moondust smells like burnt
gunpowder,' they knew what they were talking about.

To
be clear, moondust and gunpowder are not the same thing. Modern
smokeless gunpowder is a mixture of nitrocellulose (C6H8(NO2)2O5)
and nitroglycerin (C3H5N3O9).
These are flammable organic molecules "not found in lunar
soil," says Gary Lofgren of the Lunar Sample Laboratory
at NASA's Johnson Space Center. Hold a match to moondust--nothing
happens, at least, nothing explosive.

What
is moondust made of? Almost half is silicon dioxide
glass created by meteoroids hitting the moon. These impacts,
which have been going on for billions of years, fuse topsoil
into glass and shatter the same into tiny pieces. Moondust
is also rich in iron, calcium and magnesium bound up in minerals
such as olivine and pyroxene. It's nothing like gunpowder.

So
why the smell? No one knows.

ISS
astronaut Don Pettit, who has never been to the moon but has
an interest in space smells, offers one possibility:

"Picture
yourself in a desert on Earth," he says. "What do
you smell? Nothing, until it rains. The air is suddenly filled
with sweet, peaty odors." Water evaporating from the
ground carries molecules to your nose that have been trapped
in dry soil for months.

Maybe
something similar happens on the moon.

"The
moon is like a 4-billion-year-old desert," he says. "It's
incredibly dry. When moondust comes in contact with moist
air in a lunar module, you get the 'desert rain' effect--and
some lovely odors."
(For the record, he counts gunpowder as a lovely odor.)

Gary
Lofgren has a related idea: "The gases 'evaporating'
from the moondust might come from the solar wind." Unlike
Earth, he explains, the moon is exposed to the hot wind of
hydrogen, helium and other ions blowing away from the sun.
These ions hit the moon's surface and get caught in the dust.

It's
a fragile situation. "The ions are easily dislodged by
footsteps or dustbrushes, and they would be evaporated by
contact with warm air inside the lunar module. Solar wind
ions mingling with the cabin's atmosphere would produce who-knows-what
odors."

Want
to smell the solar wind? Go to the moon.

Schmitt
offers yet another idea: The smell, and his reaction to it,
could be a sign that moondust is chemically active.

"Consider
how moondust is formed," he says. "Meteoroids hit
the moon, reducing rocks to jagged dust. It's a process of
hammering and smashing." Broken molecules in the dust
have "dangling bonds"--unsatisfied electrical connections
that need atomic partners.

Right:
Moondust is formed by pounding; the "hammers" are
meteoroids. Image credit: Prof. Larry Taylor, University of
Tennessee. [More]

Inhale
some moondust and what happens? The dangling bonds seek partners
in the membranes of your nose. You get congested. You report
strange odors. Later, when the all the bonds are partnered-up,
these sensations fade.

Another
possibility is that moondust "burns" in the lunar
lander's oxygen atmosphere. "Oxygen is very reactive,"
notes Lofgren, "and would readily combine with the dangling
chemical bonds of the moondust." The process, called
oxidation, is akin to burning. Although it happens too slowly
for smoke or flames, the oxidation of moondust might produce
an aroma like burnt gunpowder. (Note: Burnt and unburnt gunpowder
do not smell the same. Apollo astronauts were specific. Moondust
smells like burnt gunpowder.)

Curiously,
back on Earth, moondust has no smell. There are hundreds of
pounds of moondust at the Lunar Sample Lab in Houston. There,
Lofgren has held dusty moon rocks with his own hands. He's
sniffed the rocks, sniffed the air, sniffed his hands. "It
does not smell like gunpowder," he says.

Were
the Apollo crews imagining things? No. Lofgren and others
have a better explanation:

Moondust
on Earth has been "pacified." All of the samples
brought back by Apollo astronauts have been in contact with
moist, oxygen-rich air. Any smelly chemical reactions (or
evaporations) ended long ago.

This
wasn't supposed to happen. Astronauts took special "thermos"
containers to the moon to hold the samples in vacuum. But
the jagged edges of the dust unexpectedly cut the seals of
the containers, allowing oxygen and water vapor to sneak in
during the 3-day trip back to Earth. No one can say how much
the dust was altered by that exposure.

Schmitt
believes "we need to study the dust in situ--on
the moon." Only there can we fully discover its properties:
Why does it smell? How does it react with landers, rovers
and habitats? What surprises await?

NASA
plans to send people back to the moon in 2018, and they'll
stay much longer than Apollo astronauts did. The next generation
will have more time and better tools to tackle the mystery.

Eric
Joneswho penned the
Apollo
Lunar Surface Journal, a must-read for historians
of the Apollo program. Many of the radio transmissions
and recollections reported in this story come from
the Journal.

Science
writer Trudy
Bell for her work on an earlier version
of this story.

Note:
As the story explains, moondust is nothing like modern
smokeless gunpowder. It's nothing like old-fashioned
"black powder" either: Black powder, an ancient
form of gunpowder, is mainly potassium nitrate (75%),
a.k.a. "saltpeter," mixed with charcoal (15%)
and sulfur (10%). Moondust contains almost none of these
ingredients. For example, "the maximum amount of
sulfur in moondust we've studied is only 0.2%,"
says Gary Lofgren of the Lunar Sample Laboratory at
NASA's Johnson Space Center. "And the explosive
ingredient of black powder, saltpeter, is completely
absent."